Cement composition

ABSTRACT

The cement composition of this invention includes 10 to 35% by weight of portland cement, 65 to 90% by weight of a synthetic pozzolan, and an activator for reducing the positive surface potential of the synthetic pozzolan. The synthetic pozzolan is obtained by an at least partial reduction from combustion residues having a SiO 2  /CaO ratio of greater than 1:1 and containing Na 2  O and K 2  O as alkali oxides in an amount exceeding 1.5% by weight. The activator includes anionically active or anionic tensides.

This application is the national phase of international applicationPCT/AT97/00256, filed Nov. 21, 1997 which designated the U.S.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a cement composition comprising

a) 10 to 35% by weight of cement, in particular portland cement

b) 65 to 90% by weight of a synthetic pozzolan obtained by an at leastpartial reduction from combustion residues having an SiO₂ /CaO ratioof >1, such as e.g. waste incineration slags, and containing alkalioxides (Na₂ O and K₂ O) in amounts exceeding 1.5% by weight,

c) an activator for reducing the positive surface potential of thesynthetic pozzolan, such as e.g. anionically active or anionic tensides,in particular, sulfonates.

2. Prior Art

Cement compositions containing cement and, in particular, portlandcement and additionally more or less large portions of extenders orfillers are known. Also has already been proposed to employ cement alongwith natural pozzolans or synthetic pozzolans as well as carbonates asextenders, the majority of such extenders or fillers being limitedmerely in view of the desired strength values. With some such mixtures,demixing by sedimentation may occur, this applying, in particular, tofly ash, sand or blast furnace slag. The risk of demixing bysedimentation is faced, in particular, with low surface charges, whereinpozzolans, fly ash, microsilica and blast furnace slag, as a rule, havelow to clearly negative surface charges. Pozzolans having clearlynegative surface charges stand out for their good miscibility withcement, in particular, in the alkaline range. The same holds formicro-silica powders exhibiting extremely negative surface charges.Investigations have demonstrated that the respectively measured negativesurface charge quite definitely is a function of the pH, wherein lowsurface charges remaining nearly constant over the total pH range weremeasured for fly ash and blast furnace slag.

Mixtures to be used for the production of coarse- and/orfine-grain-flux-containing concrete comprising 70 to 35% by weight ofcement and 30 to 65% by weight of latent hydraulic substances as well as0.05 to 1% by weight of a dispersant such as, for instance, ligninsulfonate have become known from DE-A1 31 05 054. In DE-A1 31 05 054,decreasing strength values were measured with increasing amounts ofsynthetic pozzolans.

Synthetic pozzolans and, in particular, those obtained by an at leastpartial reduction from combustion residues having an SiO₂ /CaO ratioof >1, such as e.g. waste incineration slags, and containing alkalioxides in amounts exceeding 1.5% by weight are characterized by positivesurface charges. Consequently, the admixture of such synthetic pozzolansusually has not resulted in enhanced strength properties, in particular,in terms of early strength. Due to the formation of agglomerates, mixedcement exhibits poor working properties. As expected, such cementmixtures have, therefore, strongly fallen short of the early strengthvalues measured for portland cement or cement compositions containingother extenders or fillers.

SUMMARY OF THE INVENTION

Surprisingly, it has now been found that cement composition comprising

a) 10 to 35% by weight of cement, in particular portland cement

b) 65 to 90% by weight of a synthetic pozzolan obtained by an at leastpartial reduction from combustion residues having an SiO₂ /CaO ratioof >1, such as e.g. waste incineration slags, and containing alkalioxides (Na₂ O and K₂ O) in amounts exceeding 1.5% by weight, and

c) an activator.

The strength values of the portland cement and pozzolan may be raised tovalues not only equal, but even superior to those of the best knownstandard cement if an activator is additionally added to reduce thepositive surface potential of the synthetic pozzolan, such as e.g.anionically active or anionic tensides. The high alkali oxide content asobserved, in particular, in acidic slags causes surface charging bysodium and potassium ions, pozzolanic hydration requiring from thematerial to have a relatively high density or small particle interspace.Without any activator, the positive surface charge causes the repulsionof the particles and hence a low particle density, thereby preventing orimpeding hydration and, thus, any contribution to increasing strength.Such surface charges now may be compensated for by the activatorsmentioned, causing the pozzolanic particles to no longer repel mutuallyand rendering feasible a higher density and, on the whole, an enhancedhydration. Excellent early strength values and outstanding finalstrength values could be obtained even at relatively high portions ofsuch slags or synthetic pozzolans in the cement compositions.

This effect is intensified if, as proposed by the invention, the cementportion is limited to a maximum of 35% by weight. This unexpectedobservation is substantiated by the high Na₂ O and K₂ O contents in theacidic pozzolans containing 40 to 70% by weight of SiO₂. Such amounts ofNa₂ O and K₂ O result in the catalysis of the pozzolanic reaction ofSiO₂, wherein too high a CaO or cement portion causes a decrease of theearth-alkaline activity, since the dissociation of Ca(OH)₂ and hence thehydration of the cement are repressed by the great amount of OH ionsavailable from the hydration of the alkali compounds of the pozzolan,thus reducing the catalytic activity as a whole.

In a particularly advantageous manner, the cement composition containsactivators in amounts of from 1 to 3% by weight. Preferably, pozzolansmay be contained in the cement composition provided by the invention inamounts of from 65 to 70% by weight, excellent strength propertieshaving been surprisingly found despite such high amounts of pozzolans.

In a particularly simple manner, an activator selected from the groupconsisting of organic sulfonates, for instance lignin sulfonate ornaphthalene sulfonate, may be added in amounts of from 1.0 to 2.0% byweight, preferably 1.5% by weight.

A particularly high final strength may be obtained if, as in accordancewith a preferred further development, the pozzolan is ground to afineness of 3000 to 6000 cm² /g.

The addition of anionically active or anionic tensides, in principle,may be effected already during the preparation of the cementcomposition. In a particularly simple manner, it may alternatively beproceeded in that the anionically active or anionic tensides are addedalong with water during the production of concrete or mortar.

The effect of activators is particularly striking with syntheticpozzolans having high alkali oxide contents, wherein, preferably, suchsynthetic pozzolans are used, whose Na₂ O and K₂ O content is chosen tobe larger than 2% by weight.

In the following, the invention will be explained in more detail by wayof strength measurements and properties of the employed materialsillustrated in the drawing.

BRIEF DESCRIPTION OF THE DRAWINGS

In FIG. 1, the surface charge ESA in mPa.m/V of standard portland cementis illustrated over the pH range from 10 to 11.5. From that illustrationit is apparent that standard cement has a substantially constant lownegative surface charge over the pH range in question.

FIG. 2 depicts the surface charges of a synthetic pozzolan derived fromwaste incineration slags and having an Na₂ O and K₂ O content of morethan 2.5% by weight, it being apparent that, over the pH range inquestion of 9.5 to 12, the surface charge is slightly positive andsubstantially constant.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION

In the Table below, the compressive strengths measured after one, two ortwenty-eight days for a reference standard cement, two laboratory cementcompositions and a cement composition according to the invention havebeen compared. The values indicated in column I for the reference cementare characterized by a good early strength and a good final strength.

The laboratory cement LZ1 (column II) consisted of 60% by weight of thereference cement to which 40% by weight of lime were added undergrinding. The result of the extender in that case is that the finalstrength remains markedly lower at good early strength values.

The laboratory cement LZ2 (column III) consisted of 30% by weight of thestandard cement indicated in the first column and 70% by weight of apozzolanic slag obtained by partial reduction from waste incinerationslags. The selected pozzolan was added in an overall quantity of 70% byweight, having an SiO₂ /CaO ratio of >1 and alkali oxides in amounts ofmore than 1.5% by weight. The values obtainable in this mannerdemonstrate that the early strength markedly decreases upon the additionof such pozzolanic slags.

In column IV, naphthalene sulfonate has been added to the laboratorycement LZ2 according to column III in an overall quantity of 1.5% byweight, whereby a marked improvement of the early strength and aconsiderable increase in the final strength could surprisingly beobtained.

Column IV of the Table below indicates the strength values of aniron-oxide-containing and hence only partially reduced syntheticpozzolan having the composition according to analysis A. When using amore strongly reduced and hence iron-oxide-free pozzolan according toanalysis B, an early strength higher by approximately 10% was observedat a substantially equal final strength.

The clinker used for the production of mixed cement had the followingcomposition: Blaine index: 5600 cm² /g (Blaine)

    ______________________________________                                        Loss on ignition   2.0%     by weight                                         SiO.sub.2          20.4%    by weight                                         Al.sub.2 O.sub.3   5.0%     by weight                                         Fe.sub.2 O.sub.3   2.6%     by weight                                         CaO                63.2%    by weight                                         MgO                2.1%     by weight                                         SO.sub.3           3.0%     by weight                                         K.sub.2 O          1.1%     by weight                                         Na.sub.2 O         0.13%    by weight                                         TiO.sub.2          0.29%    by weight                                         Cr.sub.2 O.sub.3   0.0096%  by weight                                         Mn.sub.2 O.sub.3   0.04%    by weight                                         P.sub.2 O.sub.5    0.09%    by weight                                         Cl                 0.01%    by weight                                         Total              99.97%   by weight                                         ______________________________________                                    

Analysis A:

Iron-oxide-containing puzzolan

(Blaine index=5600 cm² /g as well as 12% retainings on 20 μm screen)

    ______________________________________                                        SiO.sub.2         43.6%   by weight                                           Al.sub.2 O.sub.3  13.7%   by weight                                           Fe.sub.2 O.sub.3  12.4%   by weight                                           CaO               19.1%   by weight                                           MgO               2.6%    by weight                                           SO.sub.3          0.17%   by weight                                           K.sub.2 O         0.84%   by weight                                           Na.sub.2 O        2.8%    by weight                                           TiO.sub.2         1.7%    by weight                                           Cr.sub.2 O.sub.3  0.76%   by weight                                           Mn.sub.2 O.sub.3  0.47%   by weight                                           P.sub.2 O.sub.5   0.33%   by weight                                           Cl                0.03%   by weight                                           Cu, Zn            balance                                                     Total             100%    by weight                                           ______________________________________                                    

Analysis B:

Strongly reduced puzzolan

(Blaine index=5400 cm² /g as well as 16% retainings on 20 μm screen)

    ______________________________________                                        SiO.sub.2         51%     by weight                                           Al.sub.2 O.sub.3  16%     by weight                                           CaO               22%     by weight                                           MgO               3%      by weight                                           SO.sub.3          0.2%    by weight                                           K.sub.2 O         1%      by weight                                           Na.sub.2 O        3.5%    by weight                                           TiO.sub.2         2%      by weight                                           Total             98.70%  by weight                                           ______________________________________                                    

                  TABLE                                                           ______________________________________                                        Compressive strengths after 1, 2 and 28 days                                                                  IV                                                    I                       LZ2 +                                                 REF cement              1.5%                                                  NORMO4  II       III    naphatalene                                           Siggenthal                                                                            LZ 1     LZ2    sulfonate                                     ______________________________________                                        W/Z       0.46      0.53     0.45 0.30                                        1 day     14.3      15.6     6.4  10.0                                        [N/mm.sup.2 ]                                                                 2 days    27.8      21.8     9.3  24.7                                        [N/mm.sup.2 ]                                                                 28 days   55.4      28.1     32.9 75.7                                        [N/mm.sup.2 ]                                                                 ______________________________________                                         W/Z = water/binder ratio                                                 

What is claimed is:
 1. A cement composition comprising:a) 10 to 35% byweight of portland cement; b) 65 to 90% by weight of a syntheticpozzolan obtained by an at least partial reduction from combustionresidues having a SiO₂ /CaO ratio of greater than 1:1 and containing Na₂O and K₂ O as alkali oxides in an amount exceeding 1.5% by weight; andc) an activator for reducing a positive surface potential of thesynthetic pozzolan, said activator comprising anionically active oranionic tensides.
 2. A cement composition according to claim 1, whereinthe composition contains the activator in a concentration of from 1 to3% by weight.
 3. A cement composition according to claim 1 or 2, whereinthe composition contains the pozzolan in a concentration of from 65 to70% by weight.
 4. A cement composition according to claim 1 or 2,wherein the activator comprises lignin or naphthalene sulfonates presentin the composition in a concentration of from 1.0 to 2.0% by weight. 5.A cement composition according to claim 1 or 2, wherein the pozzolan isground to a fineness of 3000 to 6000 cm² /g.
 6. A cement compositionaccording to claim 1 or 2, wherein the anionically active or anionictensides are added to the composition along with water.
 7. A cementcomposition according to claim 1 or 2, wherein the Na₂ O and K₂ Ocontent of the pozzolans is larger than 2% by weight.
 8. A cementcomposition according to claim 3, wherein the activator comprises ligninor naphtholene sulfonates present in the composition in a concentrationof from 1.0 to 2.0% by weight.
 9. A cement composition according toclaim 3, wherein the pozzolan is ground to a fineness of 3000 to 6000cm² /g.
 10. A cement composition according to claim 4, wherein thepozzolan is ground to a fineness of 3000 to 6000 cm² /g.
 11. A cementcomposition according to claim 8, wherein the pozzolan is ground to afineness of 3000 to 6000 cm² /g.
 12. A cement composition according toclaim 3, wherein the anionically active or anionic tensides are added tothe composition along with water.
 13. A cement composition according toclaim 4, wherein the anionically active or anionic tensides are added tothe composition along with water.
 14. A cement composition according toclaim 5, wherein the anionically active or anionic tensides are added tothe composition along with water.
 15. A cement composition according toclaim 8, wherein the anionically active or anionic tensides are added tothe composition along with water.
 16. A cement composition according toclaim 9, wherein the anionically active or anionic tensides are added tothe composition along with water.
 17. A cement composition according toclaim 10, wherein anionically active or anionic tensides are added tothe composition along with water.
 18. A cement composition according toclaim 11, wherein the anionically active or anionic tensides are addedto the composition along with water.
 19. A cement composition accordingto claim 3, wherein the Na₂ O and K₂ O content of the pozzolans islarger than 2% by weight.
 20. A cement composition according to claim 4,wherein the Na₂ O and K₂ O content of the pozzolans is larger than 2% byweight.
 21. A cement composition according to claim 5, wherein the Na₂ Oand K₂ O content of the pozzolans is larger than 2% by weight.
 22. Acement composition according to claim 6, wherein the Na₂ O and K₂ Ocontent of the pozzolans is larger than 2% by weight.
 23. A cementcomposition according to claim 8, wherein the Na₂ O and K₂ O content ofthe pozzolans is larger than 2% by weight.
 24. A cement compositionaccording to claim 9, wherein the Na₂ O and K₂ O content of thepozzolans is larger than 2% by weight.
 25. A cement compositionaccording to claim 10, wherein the Na₂ O and K₂ O content of thepozzolans is larger than 2% by weight.
 26. A cement compositionaccording to claim 11, wherein the Na₂ O and K₂ O content of thepozzolans is larger than 2% by weight.
 27. A cement compositionaccording to claim 12, wherein the Na₂ O and K₂ O content of thepozzolans is larger than 2% by weight.
 28. A cement compositionaccording to claim 13, wherein the Na₂ O and K₂ O content of thepozzolans is larger than 2% by weight.
 29. A cement compositionaccording to claim 14, wherein the Na₂ O and K₂ O content of thepozzolans is larger than 2% by weight.
 30. A cement compositionaccording to claim 15, wherein the Na₂ O and K₂ O content of thepozzolans is larger than 2% by weight.
 31. A cement compositionaccording to claim 16, wherein the Na₂ O and K₂ O content of thepozzolans is larger than 2% by weight.
 32. A cement compositionaccording to claim 17, wherein the Na₂ O and K₂ O content of thepozzolans is larger than 2% by weight.
 33. A cement compositionaccording to claim 18, wherein the Na₂ O and K₂ O content of thepozzolans is larger than 2% by weight.